Abstract

Background

Radiation-induced hypothyroidism (RIHT) is common complication of head and neck cancer or Hodgkin disease treated with radiation to the thyroid region. However, the effect of radiation on the thyroid in breast cancer patients who received a radiation therapy to the supraclavicular (SC) field is unknown. We evaluated the prevalence of hypothyroidism (HT) in these patients.

Methods

Between April 2007 and May 2016, consecutive patients with invasive breast cancer who received SC radiation in Kobe City Medical Center General Hospital were recruited. Thyroid-stimulating hormone (TSH), free thyroxine (fT4), anti-thyroglobulin antibody (TgAb) and anti-thyroid peroxidase antibody (TPOAb) were investigated during the period of April to August 2016. Based on the radiation planning CT-images of each patient, the volume of the thyroid gland was calculated and dose-volume parameters were estimated. The primary endpoint was the prevalence of HT as determined by a high serum level of TSH and low serum level of fT4. Secondary endpoints included the prevalence of subclinical HT as determined by a high serum level of TSH and normal level of fT4.

Results

A total of 68 consecutive patients were screened. Among these patients, 26 were excluded from evaluation (10 patients died, 6 had a history of previous thyroid disease, 10 were lost to follow up) and blood samples were taken in 42 patients. One (2.4%) and six (14.3%) of these patients had HT and subclinical HT, respectively, with a mean TSH level of 8.3 µU/ml. On univariate analysis, a possible predictive factor of HT and subclinical HT was a thyroid volume < 8 cm3 (OR 6.44, 95% CI 1.13-36.6), in contrast, age, dose-volume parameters, TgAb positivity or TPOAb positivity and follow-up time were not associated with HT or subclinical HT.

Conclusions

The prevalence of HT in Japanese breast cancer patients treated with radiation to the SC region is relatively low compared with that in head and neck cancer and Hodgkin disease patients. Although thyroid volume appeared to be a predictive marker of HT or subclinical HT in this cohort, further prospective evaluation is needed.